The overall goal of the proposed research is to apply flow perfusion bioreactor culture of mesenchymal stem cells (MSCs) toward the fabrication of bioactive, biodegradable polymer/extracellular matrix (ECM) hybrid constructs for tissue engineering. The present proposal focuses upon the development and application of this innovative approach to fabricate bi-layered constructs for the repair of osteochondral defects. It is hypothesized that flow perfusion bioreactor culture of MSCs upon electrospun poly(5-caprolactone) (PCL) nanofiber scaffolds in medium augmented with osteogenic or chondrogenic supplements will produce bioactive polymer/ECM hybrid constructs with an ECM component containing osteogenic or chondrogenic factors, respectively, and that the character of the ECM is influenced by the culture conditions and the properties of the scaffold. It is further hypothesized that, following decellularization and implantation, these acellular osteogenic and chondrogenic polymer/ECM hybrid constructs will direct the differentiation of host progenitor cells toward the generation of bone or cartilage tissue, respectively. It is hypothesized that scaffolds composed of nanofibers will result in the deposition of ECM containing more osteogenic or chondrogenic factors than ECM deposited on microfiber scaffolds, as nanofiber scaffolds more closely approximate the scale of native ECM molecules and, due to the smaller pore size, produce increased shear stress at a given flow rate. The effects of the applied shear stress, the architecture of the scaffold (microfibers vs. nanofibers), and the culture conditions on the generated osteogenic and chondrogenic hybrid constructs will be investigated by monitoring the presence of molecules characteristic of the respective tissue types (e.g., collagen type I for bone and collagen type II for cartilage). Further, the culture duration for ECM generation will be modulated to examine the effect of the maturity of the ECM component of the decellularized hybrid constructs upon the osteoblastic and chondrocytic differentiation of subsequently seeded MSCs in vitro (as measured by differentiation markers such as alkaline phosphatase activity, calcium and glycosaminoglycan content, and the presence of collage types I and II) and upon tissue formation in vivo in an osteochondral defect in a rabbit model (as measured by histology and histomorphometry). Finally, acellular bi-layered polymer/ECM hybrid constructs will be fabricated with an osteogenic layer and a chondrogenic layer and then implanted in a rabbit osteochondral defect model to assess the potential of the constructs to influence the spatial differentiation of progenitor cells of the host to form bone and cartilage in the respective layers. This novel approach to fabricate acellular bioactive degradable tissue engineering constructs containing ECM rich in growth factors produced by cells under engineered conditions in vitro presents tremendous potential for application in the guided regeneration of a wide range of tissues. A significant clinical need exists for novel implant materials capable of promoting the repair and regeneration of injured or compromised tissues, such as damaged articular cartilage. Indeed, as cartilage has a limited natural capacity to repair itself, damage to articular cartilage and underlying bone often leads to considerable clinical problems that afflict million of people worldwide, including pain, limited mobility and osteoarthritis. The research project presented in this proposal seeks to apply advanced cell culture technologies to fabricate biologically active implant materials that can promote cells within the recipient to regenerate or repair specific damaged tissues, in this case articular cartilage and underlying bone.